Although the electrolyte of a lithium ion battery is generally liquid, a gel or solid electrolyte may be used as a polymer electrolyte. The following characteristic properties are required for all of these electrolytes.    1) They must exhibit high conductivity (high movability of a lithium ion).    2) They must exhibit great chemical and electrochemical stability for electrode materials.    3) They must have a wide usable temperature range.    4) They must have high safety.    5) They must be inexpensive.
An aprotic organic solvent having high specific dielectric constant and low viscosity is suitable as an electrolyte solvent for obtaining high conductivity, which contains a lithium salt in a high concentration. However, since a solvent having high specific dielectric constant and strong polarity has high viscosity, a mixture of solvents is used in an actual electrolyte. For example, there is known a mixed solvent of propylene carbonate (PC) having a dielectric constant of 64.4 and a viscosity of 2.3 cp or ethylene carbonate (EC) having a dielectric constant of 95.3 and a viscosity of 1.9 cp and dimethyl carbonate (DMC) having a viscosity of 0.59 cp. This mixed solvent has composition which shows maximum conductivity, and this composition is investigated in detail together with the type of an electrolyte salt to be added.
As the electrolyte salt are used lithium perchlorate (LiClO4), and LiPF6, LiAsF6, Li(CF3SO2)2N, LiBF4 and LiCF3SO3 all of which contain fluorine. The ion conductivity of an organic solvent electrolyte obtained by dissolving one of these electrolyte salts is about 10−2 S/cm.
A gel electrolyte which is a mixture of an organic polymer and a liquid electrolyte has high ion conductivity and was commercialized earlier than a solid electrolyte. A carbon-based negative-electrode material is used in the gel electrolyte to manufacture a lithium ion secondary battery, especially a thin film battery.
Heretofore, a low-molecular weight gelling agent or a polymer gelling agent has been used to solidify organic liquids such as animal and vegetable oils, esters, polyols, ethers, alcohols and hydrocarbons in industrial processing fields such as coating compositions, inks, lubricant oils, agricultural products, marine products, cosmetics, medical goods, fibers, resins, polymers, rubbers and metals. The polymer gelling agent is often used in the technical field of batteries as a gelling agent for gel electrolytes.
The low-molecular weight gelling agent was developed after the above polymer gelling agent, and 12-hydroxystearic acid, dialkylurea derivatives and dibenzylidene sorbitol are known as the low-molecular weight gelling agent.
Although 12-hydroxystearic acid out of these is inexpensive, the number of kinds of organic liquids which can be gelled by this is small, and the softening temperature of the obtained gel is low. The number of kinds of organic liquids which can be gelled by dialkylurea derivatives is also small. Although a strong gel is formed by the addition of a small amount of dibenzylidene sorbitol, it has a defect that it liberates benzaldehyde and is not suitable for solidifying a short-chain alcohol having a low boiling point because it has a high melting point. Alkali metal salts and alkali earth metal salts of a fatty acid must be added in large quantities for gelation or solidification and their usable conditions are limited.
It has been reported that a perfluoroalkylalkane represented by the formula F(CF2)n(CH2)nH in which n is 12 and m is 8 to 20 gels decane and that a perfluoroalkylalkane of the above formula in which n is 10 and m is 12 gels a hydrocarbon solvent (refer to Robert J. Twang, et al, “Observations of a “Gel” Phase in Binary Mixtures of Semifluorinated n-Alkanes with Hydrocarbon Liquids” Macromolecules 1985, 18, 1361-1362).
It has also been reported that a perfluoroalkylalkane represented by the formula F (CF2)8(CH2)8H gels methyl alcohol, ethyl alcohol and propyl alcohol (refer to Massimo Napoli, et al, “Synthesis of F(CF2)8(CH2)8H and gel phase formation from its solutions in homologous alcohols).
However, since the kinds of organic liquids which can be gelled by the above prior art gelling agents are limited, a large amount of each of the gelling agents is required for gelation, an organic low-molecular weight gelling agent which can gel many kinds of organic liquids with a small amount has been unknown up till now. An organic low-molecular weight gelling agent which gels a high-dielectric constant solvent suitable for organic electrolytes has been unknown as well.